The IEEE 802.3af and 802.3 at specifications, also known as Power over Ethernet (PoE), provides a framework for delivering DC power concurrently with data over standard Ethernet cabling. A PoE system includes three basic components: power sourcing equipment (PSE) for providing power, a powered device (PD) for receiving and consuming the power, and cabling for transferring the power from the PSE to the PD. The PSE, as defined by the IEEE 802.3af/t specifications, performs much of the basic power provisioning process, including detection, classification, operation, and disconnection.
Detection is first performed by the PSE to determine if a valid PD is connected to its power providing output. Detection is carried out by inducing a small voltage at the output of the PSE to detect a specific 25 KΩ signature resistor. This signature indicates that a valid PD is connected and that the provision of power to the PD can begin.
After a valid PD is detected, an optional classification stage can be performed to estimate the amount of power required by the PD. To perform classification, the PSE again induces a voltage around 15.5-20.5 Vdc for a period of time within 10 to 75 ms. The current consumed by the PD during this time period indicates to the PSE its power classification.
Following detection and optional classification, the output power of the PSE can be increased, during the operation stage, to its full voltage capacity, which is typically around 48 Vdc. The output voltage of the PSE is gradually increased to its full voltage capacity to prevent high frequency noise from disrupting data being transferred concurrently with the power.
The final stage of the power provisioning process involves removal of power following the disconnection of the PD connected to the PSE. The IEEE 802.3af/t specifications define two specific techniques for power disconnection; namely, DC disconnect and AC disconnect. Both methods provide the same desired result—the detection of a disconnected PD and the removal of power within 300 to 400 ms thereafter. The removal of power when a PD is disconnected is important because the PD may be replaced by a non-PoE-ready device, which may result in damage.
DC disconnect is performed at the PSE by measuring the current consumed by the PD. If the PD is disconnected at any point, the consumption of current by the PD would cease, indicating disconnection of the PD. AC disconnect, on the other hand, entails the addition of a low AC signal on top of the 48 Vdc operating voltage. The returned AC signal amplitude is monitored at the PSE. While the PD is connected, the low impedance of the PD lowers the returned AC signal. During disconnection, however, the AC signal level will increase, indicating disconnection of the PD.
In conventional PSEs implementing AC disconnect, a diode is used to isolate the DC source, providing the 48 Vdc operating voltage, from the AC disconnect signal. Depending on the type of diode utilized, the diode can have a forward voltage drop of 0.3-0.7 Vdc at 600 in mA, or a total power consumption around 0.2-0.4 W, for example. Not only does the isolation diode increase overall power consumption, but further increases the overall temperature at the media dependent interface (MDI) of the PSE. This excess power consumption and temperature becomes even more apparent in multi-port hubs or switches that are PoE-ready. For example, in a 24-port hub that is PoE-ready, 24 separate diodes (one for each port) can be required to isolate the DC supply from the AC disconnect signal(s).
Therefore, what is needed is an apparatus for isolating a DC supply of a PSE from an AC disconnect signal, while limiting any additional power consumption and heat produced as a result thereof.
The present invention will be described with reference to the accompanying drawings. The drawing in which an element first appears is typically indicated by the leftmost digit(s) in the corresponding reference number.